Figures

Figure 1.System design: (a) the optical unit is the NIMO CW-NIRS oxymeter, (b) the electrical unit includes the custom built analog amplifier and filter and the A/D converter, (c) visual stimulation and electro-optical recording interface is provided by an LCD projector, projection screen, optical head (Figure 2) and EEG-cap (Figure 3), and (d) the control unit is a personal computer running the supervisor software. The optical and electrical units controlled by the personal computer are both used to monitor the visual cortex. The same personal computer is used to control the LCD projector which generates the visual stimuli.

Figure 3.EEG-cap and the relative electrodes locations. The VEP signal is recorded from O1 and referred to Cz a), whereas A1 is the ground reference according to the standard 10–20 system positions for electrodes b).

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Figure 4.Experimental protocol adopted for visual stimulation. During each stimulation epoch, the windmill pattern is reversed eight times per second.

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Figure 5.Oscillatory changes in Δ[HbO2] together with the corresponding power spectral analysis for one subject, typical of the whole group. A pronounced peak at the heart rate frequency around 1 Hz is clearly visible. Around the stimulation frequency, i.e. 8 Hz, both spectra are essentially featureless.

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Figure 6.Windmill pattern stimuli and the corresponding hemodynamic responses, as acquired by the CW-NIRS system. Each line of the figure corresponds to different stimulus-contrast levels: 1%, 10% and 100%. The gray regions represent the stimulation epochs.

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Figure 7.The right column shows typical VEP signals, referred to a single subject, recorded after stimulation at increasing contrast (1, 10, 100%). The left column contains VEPs recorded in a "without stimulus" condition: the subjects gaze at a uniform unpatterned screen. The large vertical arrows indicate the correspondence between the visual stimulation protocol and the recording duration, which lasts for 60 seconds. Each 60-second interval corresponds to the interval reported as a white/grey vertical strip in Figure 6.

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Figure 8.〈Δ[HbO2]〉 (solid red circle) and 〈Δ[HHb]〉 (solid blue square) averaged over observers as a function of stimulus contrast. The bars represent the standard errors (SE) evaluated over the nine subjects whereas the dashed lines represent the best logarithmic fit of the data.

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Figure 9.〈 VEPrms〉 (solid circle) averaged over observers as a function of stimulus contrast. The bars represent the standard errors (SE) evaluated over the nine subjects whereas the dashed line represents the best logarithmic fit of the data.

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Figure 10.〈Δ[HbO2]〉 (solid red circle) and 〈Δ[HHb]〉 (solid blue square) as a function of 〈 VEPrms〉. The bars represent the standard errors (SE) evaluated over the nine subjects whereas the dashed lines represent the best linear fit of the data. Very small SE may be hidden by plot symbols.